The present invention relates to a method and apparatus for processing contaminated wash waters.
In nuclear plants, particularly nuclear power plants, waste water which contains impurities is present at many places. These impurities must be removed from the waste water before the waste water can be recirculated or before it can be discharged to the environment. These impurities are usually radioactive and are either bound to solid components contained in the water or to ionically dissolved substances. The plant which treats such waste water is called a waste water processing plant system, or simply, a a processing plant. Such waste water processing plants, are described, for example, in Atomwirtschaft, 1968, page 149, FIG. 6; in an article entitled "Abfallbehandlung" (in translation-- WASTE TREATMENT) which appeared in Atomwirtschaft, Nov. 1965, pages 624-626, especially paragraph 2 of page 626; and in the collection "Power Plant and Environment 1973", published by VGB-- Technische Vereinigung der Grosskraftwerksbetreiber e.V. (in translation-- ASSOCIATION OF OPERATORS OF LARGE POWER PLANTS).
For economical reasons, and as described, for example, in the article "Abfallbehandlung" in the Nov., 1965 issue of Atomwirtschaft, processing of radioactive liquid wastes, for instance those occurring in nuclear power plants, is usually effected in three processing lines:
1. Filtration through mechanical filters with the filtrate being then passed through ion exchangers;
2. Concentration in evaporators;
3. Filtration only through mechanical filters.
The first-listed processing line is used for the waste waters from the nuclear cooling system and from the condensation system (these waters make up 60% to 70% of the total waste water load).
The second-listed processing line is used for sump waters, laboratory waste waters, and decontamination waters from the entire control region (about 20% to 30% of the total waste water load).
The third listed processing line is used for cleaning wash waters from washing machines, showers, and hand-washing basins, as well as inactive laboratory waters from the control region (about 5% to 10% of the total waste water load).
From these water-processing steps and other cleaning operations large amounts of radioactive concentrates arise. Thus, for example, the impurities that are removed by the filters and evaporators collect in the filters and evaporators as concentrates or residues which generally contain a liquid component. Efficient service organizations for collecting and treating these concentrates do not exist. A storing of these concentrates in liquid form is presently not possible. The processes used in Europe for handling these concentrates involve the use of additive materials, such as bitumen, concrete, oil shale ash, and bone size. These add to the total volume of the final material to be disposed of. They have often required too great of a capital investment.
The storing of these concentrates for the purpose of allowing radioactive decay to run its course requires considerable capital expense, especially where waste quantities are large, such as in the case of large power plants. It therefore becomes worthwhile to dewater and solidify such concentrates.
In efforts thus far made toward dewatering and solidification, a number of problems have arisen which have to this time prevented widespread acceptance. Among the problems are the following:
1. The concentrates are very variable in their compositions. A primary problem here is that waste from resin-bead ion exchange filters cannot be dewatered with a usual filter-cake-producing filter, because the resin beads as a result of their shape do not remain lying on the filter cloth or septum especially once their moisture content has sunk below a certain level. Because of this problem, filter-cake-producing filters have been automatically dropped from consideration whenever concentrate-containing resin beads must be dewatered.
2. The concentrates have very different activity loads.
3. The dewatered and solidified material must satisfy current governmental requirements for the storage of radioactive wastes.
4. The packaging must meet requirements for the transport of dangerous materials as set for international railroad freight traffic.
The previously-noted parent application Ser. No. 119,339, now U.S. Pat. No. 3,773,177, discloses a method for treating various radioactive concentrates or residues containing liquid components, which concentrates or residues are separately produced in a nuclear processing plant in an evaporation concentrator, in resin-bead ion exchange filters and in at least one further separator, including, for example, mechanical filters, settling vessels and/or powdered-resin ion exchange filters. According to the parent application Ser. No. 119,339, now U.S. Pat. No. 3,773,177, residues from the resin-bead ion exchange filters are mixed with those of the further separator stage and are dewatered in a residue filter, while residue from the evaporation concentrator is dewatered directly in transporting and storage vessels to the dryness required for storage. A relatively small volume of residue is obtained by this method without any additional materials.
When dealing with concentrates containing used resins, chemicals can be employed to reduce the volume of the used resins but use of chemicals would result in dumping a substantially larger amount of radioactive waste water (water having a high conductivity) e.g., into streams, and would increase the cost for process components and chemicals.
As has been previously mentioned, among the waste waters which must be processed in the waste water processing plant are the wash waters. These wash waters include the waters from showers that people in the plant use after they have worked in the plant, from the hand wash basins that people use to wash their hands and from washing machines that wash the contaminated clothing of the persons working in the plant.
In the past, two methods with different degrees of efficiency have been used to process these waste wash waters. In the first method, the wash waters have been concentrated in evaporators in accordance with the second-listed processing line described above. In the evaporation method, it is presently possible to attain residual activities in the distillate at the power plants of about 10.sup..sup.-5 Ci/m.sup.3 pretty much independent of the initial activity. An example of the use of evaporators for wash waters is shown in FIG. 6 of the above-mentioned 1968 issue of Atomwirtschaft. In the second method, the waste wash waters are processed in accordance with the third-listed processing line described above in which there is a filtration method only through mechanical filters. These mechanical filters contain auxiliary filtering aids, such as kieselgur. When using this filtration method, an activity of between 10.sup..sup.-3 and 10.sup..sup.-4 Ci/m.sup.3 can be attained depending on the initial activity.
The evaporation method for processing waste wash waters is practiced with the aid of special decontamination evaporators and is very advantageous with respect to its purifying effect. However, it has a number of economical and operational drawbacks when employed for processing waste wash waters. These drawbacks include: (a) high consumption of heating steam or, when the condensation heat of the vapors is utilized, high investment costs; (b) relatively small throughput since the size of the plant is kept as small as possible because of the high investment costs involved; (c) development of a preconcentrate which contains radioactivity and which must be processed in a subsequently connected separate system until it has become a packable dry residue; and (d) operational problems in the evaporation of the wash water caused by the formation of foam from the detergents, clogging of the heating registers and control devices and valves by lint and crusting as well as a required chemical cleaning of the apparatus.
The filtration method for processing waste wash waters is still used quite often but does not operate very efficiently and has a number of drawbacks. First, the filtration method usually provides an insufficient decontamination effect (decontamination factor 10), since only the activities bound to solid matter are removed from the water. The activity bound to dissolved substances remains in the filtrate. The decontamination efficiency is particularly inefficient at the higher general activity level that occurs during the change of fuel elements and with the special generally occurring particular isotope combination of the wash water. Second, there is a poor filterability with and therefore high consumption of filtering aids resulting in a corresponding high cost of operating agents and relatively large filter surfaces. The filtering speed is usually less than 1 m.sup.3 /m.sup.2 /h.
Additionally, the previously employed filtering aid kieselgur is not too well suited for filtration of concentrated warm wash waters from washing machines because it fails to maintain a stable structure under prevailing conditions. Further, there generally is an excess of detergent in such a wash water and soapy, flaky and fibrous components require the amount of kieselgur to be added during filtration to be very high and thus the quantity of active residues to be treated in the concentration processing system is increased by a high percentage.